Digital multimedia capabilities can be incorporated into a wide range of devices, including digital televisions, digital direct broadcast systems, wireless communication devices, wireless broadcast systems, personal digital assistants (PDAs), laptop or desktop computers, tablet computers, digital cameras, digital recording devices, video gaming devices, video game consoles, cellular or satellite radio telephones, digital media players, and the like. Digital multimedia devices may implement video coding techniques, such as MPEG-2, ITU-H.263, MPEG-4, or ITU-H.264/MPEG-4 Part 10, Advanced Video Coding (AVC), to transmit and receive or store and retrieve digital video data more efficiently. Video encoding techniques may perform video compression via spatial and temporal prediction to reduce or remove redundancy inherent in video sequences. New video standards such as high efficiency video coding (HEVC) standard, which is being developed by a Joint Collaborative Team on Video Coding (JCT-VC) established by the ISO/IEC Moving Picture Experts Group (MPEG) and ITU-T Video Coding Experts Group (VCEG), continue to emerge and evolve.
In video encoding, the compression often includes spatial prediction, motion estimation, and motion compensation. Intra-coding relies on spatial prediction and transform coding, such as discrete cosine transform (DCT), to reduce or remove spatial redundancy between video blocks within a given video frame. Inter-coding relies on temporal prediction and transform coding to reduce or remove temporal redundancy between video blocks of successive video frames of a video sequence. Intra-coded frames (“I-frames”) are often used as random access points as well as references for the inter-coding of other frames. I-frames, however, typically exhibit less compression than other frames.
For inter-coding, a video encoder performs motion estimation to track the movement of matching video blocks between two or more adjacent frames or other units of video information that include encoded video blocks, such as slices of frames. Inter-coded frames may include predictive frames (“P-frames”), which may include video blocks predicted from a previous frame, and bi-directional predictive frames (“B-frames”), which may include video blocks predicted from a previous frame and a subsequent frame of a video sequence. The terms P-frames and B-frames are somewhat historic in the sense that early coding techniques limited prediction in specific directions. Newer coding formats and standards may not limit the prediction direction of P-frames or B-frames. Thus, the term “bi-directional” now refers to prediction based on two or more lists of reference data regardless of the temporal relationship of such reference data relative to the data being coded.
Consistent with newer video standards such as ITU H.264, for example, bi-directional prediction may be based on two different lists which do not necessarily need to have data that resides temporally before and after the current video block. In other words, B-video blocks may be predicted from two lists of data, which may correspond to data from two previous frames, two subsequent frames, or one previous frame and one subsequent frame. In contrast, P-video blocks are predicted based on one list, i.e., one data structure, which may correspond to one predictive frame, e.g., one previous frame or one subsequent frame.
For P- and B-video blocks, motion estimation generates motion vectors, which indicate the displacement of the video blocks relative to corresponding prediction video blocks in predictive reference frame(s) or other reference unit(s). Motion compensation uses the motion vectors to generate prediction video blocks from the predictive reference frame(s) or reference unit(s). After motion compensation, a residual video block is formed by subtracting the prediction video block from the original video block to be coded. The video encoder usually applies transform, quantization and entropy coding processes to further reduce the bit rate associated with communication of the residual block. I- and P-units are commonly used to define reference blocks for the inter-coding of P- and B-units.